This application relates to a new and unexpectedly synergistic insecticidal composition containing the commercial insecticides fenvalerate and phosmet, and particularly containing these compounds in a weight ratio of from about 20:1 to about 100:1, and formulated for application at a rate of 0.5 to 1.0 lb./acre (0.6 to 1.12 kg./ha) phosmet and 0.01 to 0.025 lb./acre (0.0112 to 0.28 kg./ha) fenvalerate. Rates of application as given herein are expressed in pounds of the active ingredient or ingredients (a.i.) per acre and the equivalent in kilograms a.i. per hectare.
Fenvalerate is the generic name of the commercial insecticide cyano(3-phenoxyphenyl)methyl-4-chloro-alpha-(1-methylethyl)benzene-acetate , which has the formula ##STR1## and is sold under serveral trademarks, including the registered trademarks Pydrin and Sumicidin. Phosmet, or N-(mercaptomethyl) phthalimide S-(O,O-dimethylphosphorodithioate) which has the formula ##STR2## is a commercial insecticide which is sold under several trademarks including the registered trademarks Imidan and Prolate.
Phosmet in particular has been in commercial use for some years and has been registered for use against a number of insects. Included among these insects are three which attack potatoes: the Colorado potato beetle, the potato flea beetle and the potato leafhopper. Registration information for use against these insects directs the application of phosmet at a rate of 2 lb./acre (equivalent to 2.24 kg./ha). Recently, however, some resistance has been shown to phosmet by these insects, particularly in certain areas of the eastern United States. The invention as described above however, appears to provide protection against the Colorado potato beetle utilizing application rates of phosmet of less then 2 lb./acre in combination with fenvalerate in the amounts indicated. Synergistic results have been obtained utilizing mixtures of phosmet and fenvalerate at weight ratios of from 20:1 to 100:1 (with a weight ratio of 50:1 being particularly preferred both for technical and economic reasons) and which is applied to a locus to be treated at a rate of 0.5 to 1.0 lb./acre (0.6-1.12 kg./ha) phosmet and 0.01 to 0.025 lb/acre (0.0112-0.028 kg./ha) fenvalerate.